Now in its sixth season, LAW & ORDER (NBC, Wednesdays, 10-11 p.m.) remains one of television's most solid dramas, despite--or is that because of?--its ever-changing cast. When the series premiered in the fall of 1990, it starred George Dzundza and Chris Noth as weather-beaten New York police detectives, and Michael Moriarty and Richard Brooks as world-weary prosecutors for the district attorney's office. Five years later, there's a whole new crew. Jerry Orbach is in his fourth season as Lennie Briscoe, the old-pro cop with the basset-hound eyes. And Lennie has a brand-new partner, Reynaldo "Rey" Curtis, played by young, granite-jawed Benjamin Bratt. On the lawyer side, it's now Sam Waterston and Jill Hennessy who are barking "Objection!" and petitioning judges for maximum sentences.

Law & Order's initial claim to distinction was its structure: The first half of every episode followed the investigation of a crime; the second half showed us the trial for that crime. I think it's the show's adherence to this game plan that has allowed for the shifting of cast members without the confusion or alienation of viewers. You may not know who's going to be the star at the start of any season, but you can depend on the cozy familiarity of the way an L&O drama will be played out.

To be sure, there have been cast changes I lament. Carolyn McCormick still pops up from time to time as the DA office's criminal psychiatrist, Dr. Elizabeth Olivet. Her sloe-eyed intelligence, though, deserves more, not less, screen time. And S. Epatha Merkerson is fine as the cops' boss, Lieut. Anita Van Buren, but I still miss the understated sarcasm and hard-bitten integrity Dann Florek brought to the role in the 1990-93 episodes. Most of all, I miss Noth, who was a real find as a TV actor. Good-looking without being a mere hunk, brooding without being merely sulky, he always managed to suggest a hot temper and a complex private life while delivering the show's patented calm, simple dialogue. Thank goodness the producers have kept Steven Hill on as dependably crusty DA Adam Schiff.

Of the newer hires, Bratt is the most problematic. On paper, his Rey Curtis sounds intriguing: a Latino cop, a practicing Catholic, happily married with three young daughters, and just inexperienced and cocky enough to clash with Lennie. But so far, Bratt has chosen to play Rey as a humorless, sobersided square.

L&O's ratings are stronger now than they've ever been, but I've noticed a drop in the quality of the writing. Too many cases derive their initial inspiration from real events (the Susan Smith case, and even, recently, that of Patty Hearst), with slight, facile alterations. Then too, the dialogue, once a model of hard-boiled terseness, is all too often now a jumble of gabby cliches.

Lennie and Rey have exchanges intended to convey how tough they are, but instead they come off sounding corny ("Ya kidnap a kid from a church? How many Hail Marys does that get ya?"; "Somehow I don't think this guy was counting"). And Waterston's Jack McCoy offers puffy, op-ed-page bromides such as "The Oprahfication of America ended when the Menendez brothers weren't convicted. The pendulum has swung, Adam. People don't care about why anymore--they care about what." What?

But you never know when L&O will let loose a corker, such as a November episode about the murder of the editor of an online magazine called BiteHead. It lampooned the absurdities of cyberspeak, had a suspenseful mystery plot, and featured a terrific performance by one of the slew of New York theater actors--in this case, Peter Frechette--that L&O employs to give the show a fresh look. When L&O clicks into place like that, it's as good as the best of TV.